Titolo della tesi: Numerical and experimental investigation on the behaviour of suction anchors in soft clay under sustained loading
Suction caissons are established as a reliable anchoring system to moor a range of offshore floating facilities in medium to ultra-deep waters, for their relatively low cost, ease installation and decommissioning, as well as lower impact on the marine wildlife, if compared to alternative systems. Suction caissons can be subjected to a combination of loadings, vertical or inclined, according to the mooring system adopted, that can be distinguished in permanent, quasi-static, and cyclic. The quasi-static, or sustained, loads, despite their oscillatory nature, differ from the cyclic loads for having generally long durations of days or even weeks. Under such loadings the caisson capacity to withstand uplift loads, relying on suction pressures induced at the base and within the soil plug, becomes questionable, due to the dissipation of excess pore pressure over time. Indeed, as a consequence of the change of soil drainage response, a reduction of the holding capacity and an accumulation of the caisson displacement are expected over time. The scarcity of literature studies addressing this fatigue problem has often led to a highly conservative and uneconomic design of such offshore foundations; this has basically motivated the present study, where an investigation of the response of suction caissons in soft clay subjected to sustained vertical uplift loads is presented. The study is based on the results of a set of small-strain FE simulations based on coupled pore fluid diffusion/stress analyses. The results of centrifuge tests are also presented and used to validate the findings of the numerical study. The first part of the research focuses on the physical interpretation of the mechanical and hydraulic interaction at the contact between the caisson lid and the soil plug, where a thin layer of water is present, as well as on the one-dimensional model, based on displacement compatibility and continuity of the water mass equations, specifically developed to describe such an interaction. The model, encoded in a Fortran-based UAMP subroutine, is used in an incremental FE procedure to simulate the progressive sliding failure of a caisson under sustained loads. The results are validated against available literature solutions, showing very good agreement, and compared to those obtained from a simplified and more efficient numerical approach. Such an approach is adopted in the second part of the research, where the possible failure mechanisms of caissons under sustained loads are investigated for different combinations of the caisson geometry, soil hydraulic and mechanical parameters, as well as soil-caisson contact strength. The results are presented in the form of failure envelopes in the load-time plane, correlating the sustained load level to the corresponding limit holding time, for each failure mechanism. A theoretical formulation of the envelopes, as well as a calculation procedure to predict the cumulated displacement of the caisson, are presented and compared to the experimental results, showing satisfactory agreement. The findings of this study are believed to be useful for pre-design purposes, especially in cases in which the sustained loads govern the design of suction anchors, what actually happened for some floating facilities in the Gulf of Mexico.